The invention relates generally to extender-filled thermoplastic compositions. More particularly, the invention relates to a polymer composition, such as a thermoplast, that can be highly loaded with low cost fillers and that can be used for manufacturing articles of high flexibility, strength, and adhesive properties suitable for many critical applications.
In the last decades, the number of thermoplastic materials have increased not so much through development of new polymers, but rather through modification of existing ones. By blending two or more different polymers, and/or by adding modifiers and fillers it becomes possible to modify such properties of articles produced from the aforementioned polymer compositions as strength, resistance to UV, resistance to oxidation, shrinkage, electrical conductivity, adhesive properties, and cost. For example, some inexpensive extender filler can increase the strength of the polymeric products and decrease their cost. All too often, however, while the targeted properties improve, some other properties deteriorate beyond the level acceptable for the given application.
It is a widely known fact that high density polyethylene and polypropylene, the materials which are easy to extrude and mold, have high shrinkage, from 2.5 to 5%, and a large coefficient of linear thermal expansion. The coefficient of linear expansion for polypropylene between 0 and 100xc2x0 C. is from 1xc3x9710xe2x88x924/xc2x0 C. to 2.5xc3x9710xe2x88x924/xc2x0 C., and for polypropylene at 20xc2x0 C. is 1.1xc3x9710xe2x88x924/xc2x0 C. That makes these materials less applicable for products which require high dimensional accuracy. However, it is also known that filling of polymers usually reduces their shrinkage and the coefficient of thermal expansion.
The filler material may be in the form of particles, flakes, fibers, etc. In the context of the present invention, the word xe2x80x9cparticlexe2x80x9d means any particulate element such as a sawdust particle, a finely cut fiber, a flake of mica, etc.
The importance of fillers in the plastic industry is to some degree reflected by the fact that production of fillers has been growing at a faster pace than that of plastics in general. Inexpensive extenders having substantially microsphere-like particles, such as calcium carbonate, reground post-industrial and post-consumer plastics (hereinafter referred to either as xe2x80x9cused plasticsxe2x80x9d or xe2x80x9crecycled plasticsxe2x80x9d), are commonly used as polymer fillers. Extrapolating from the statistics for 1967 to 1985 (provided by Kiln and Co.), the current total annual US use of extender fillers could be above 2,000,000 tons. Other sources have higher estimates.
U.S. Pat. No. 3,830,776 of August 1974, issued to Carlson and Banks, discloses a method for separating particulate fractions from fly ash for use as a filler for epoxy. The compositions formed by this method have high crush resistance and strength, but are relatively rigid.
U.S. Pat. No. 4,294,750, issued to Klingaman and Ehrenreich in October 1980, discloses a method for separating filler particles from coal-burning power plants"" fly ash for use in a nylon-based composition. These pyroplastoid particles are ellipsoidal in shape and, by weight, at least 90% of them are less than 25 xcexcm in size.
In 1986, Electric Power Research Institute (EPRI) in Palo Alto, Calif., published a study (EPRI CS4765, Project 2422-11), Evaluation of Plastic Filler Applications for Leached Fly Ash. The study looked into the commercial potentials of acid-leached fly ash from coal-burning power stations. Their conclusion was that the leached fly ash has a potential as a filler for polymeric compositions. They also concluded that the higher the ash content in a polypropylene or nylon composition, the more drastically the elasticity of the composition drops. That deficiency explains why such an inexpensive and widely available extender, such as fly ash is not widely used in the polymer industry. Instead, much of the fly ash, a plentiful byproduct of power generation, is currently disposed of in landfills at a considerable expense to coal-burning power stations.
U.S. Pat. No. 5,308,693 of May 1994, issued to Ryle et al, relates to Unstretched Synthetic Papers and Methods of Producing Same and discloses a non-stretched synthetic paper having 10 to 25 wt. % diatomaceous earth-filled and high density polyethylene-based compositions. Synthetic paper is increasingly being used in countries where wood is expensive. Since synthetic paper does not produce dust or tear easily, it has additional value for the high speed printing industry. Despite the advantages of synthetic paper, its cost is still too high to compete with wood-pulp paper in countries having abundant wood resources.
To improve the bond between a filler and the rest of the polymer composition, various so-called xe2x80x9ccoupling agentsxe2x80x9d are empirically selected, and fillers are treated with them either prior to being mixed into the composition, or during in-line compounding. For example, it is known that treatment of fine-grain fillers with silicone hydrates (SinH2n+2) maximizes the hydrophobicity and optimizes the electrical properties of filled polymer compositions; it is known that preliminary treatment of a filler with the hydrates of Si or Ti will improve the impact strength, thermal resistance, water stability, and strain-stress properties of filled compositions.
Atactic polypropylene has been known to increase the levels of loading compositions with carbon black and flame retardants (see U.S. Pat. No. 4,425,262 of January 1984). Atactic polypropylene has also been used as a thermally-removable binding agent for ceramics (see U.S. Pat. No. 5,256,609 of October 1993, issued to Dolhert). This invention relates to the clean burning green ceramic tape cast system using atactic polypropylene binder. However, such compositions are not suitable for molding into rigid components that maintain some flexibility.
While the extender fillers increase the modulus of elasticity of a composition, they drastically reduce its elongation at rupture. For any product which incorporates flexing elements or which has to withstand even occasional surface impacts, such as containers, furniture, instrument housings, automotive oil pans, bumpers and body panels, this reduced elongation is detrimental. Thus, for all practical purposes these filled polymers cannot be effectively used for items requiring stretching or bending properties.
Table 1 shows the change of some properties for a selected group of polymer compositions loaded 30 to 40 wt. % (percentage in weight) with various substantially microspheric fillers.
The data given in Table 1 are typical; highly extender-filled compositions have low relative elongation and do not have high tensile strength. The actual data related to filled polypropylene, polyamide 6,6, and polyvinylchloride are taken from Plastics Compound, 1986, v. 9, no. 7, pp. 12-18. The actual data about calcium carbonate-filled high density polyethylenes marketed by Washington Penn and RTP come from Plastic Technology, 1992/1993, pp. 700-701. Unfortunately, I can only guess about the properties of the high density polyethylenes used by Washington Penn and by RTP in making their calcium carbonate-filled polymers. That, however, doesn""t change things in principle: compositions on the market which have high elongation and high tensile strength. Whereas high density polyethylene has usually over 500% elongation at yield and over 20 MPa of tensile strength, RTP-740 has only 11 % maximum elongation and 17 MPa of tensile strength, and PE-3CC-3 has even less elongation and only 9 MPa of tensile strength.
The author of the present patent application has studied the nature of border regions between the filler particles and the surrounding matrix materials. The study revealed that the aforementioned border zones have microcracks which can be detected under an electron microscope. FIG. 1 illustrates a filler particle 10 in continuous polymer matrix 12 of a filled polymer and schematically represents a slice of a fly ash-filled HDPE specimen obtained on an electron microscope; the actual size of filler particle 10 is about 12 xcexcm in diameter. A dark xe2x80x9cdonutxe2x80x9d-like zone 14 around fly ash particle 10 in FIG. 1 is the aforementioned zone of embryonic microcracks. These embryonic microcracks can be attributed to a combination of the inner stresses due to the shrinkage of polymer matrix material 12, and the stresses produced by the slicing of the specimen in preparation to its viewing in the microscope. It is obvious that such compositions which readily develop embryonic microcracks cannot be flexible or strong.
Attempts have been made to solve the above problems for filled high density polyethylene. For example, the composition disclosed in application Ser. No. 3,740,522 for USSR Inventor""s Certificate, filed on May 17, 1984, by the author of the present patent application, solves a problem about the low elongation, but the flex modulus and the tensile strength still remain at a very low level. Thus, an average tensile strength at yield ensured by the compositions described in the aforementioned USSR Application for inventor""s certificate does not exceed 21 MPa.
Among polymers, high density polyethylene (HDPE) is considered to be a good material for anticorrosive protection because of its relatively low cost, relatively low permeability to moisture and other oxidizing agents, and sufficient ability to elongate and to withstand occasional surface impacts. HDPE is also considered to be an effective erosion resistant polymer. However, the effectiveness of HDPE as an anticorrosive coating material for the outside of metal pipes, as well as an antierosion coating material, e.g., to protect the inner surfaces of metal gas pipes against erosion due to the effect of the solid particles transported in pipes with the flow of gas, are significantly reduced by its very low adhesion to metals. One of the reasons for that, but not the only one, is that HDPE has very high shrinkage. Therefore, when HDPE is used as a corrosive layer in a critical application, it is usually used as the upper layer in a sandwich of anticorrosive coatings. Here is a sample of such a three-layer sandwich: a 0.6 mm thick layer of epoxy to get a better adhesion with the metal; a 0.5 mm tie layer to facilitate a reasonable bond between the epoxy and the HDPE; and a 1 mm thick layer of HDPE to shield the other two layers of the sandwich from penetration of oxidizing substances, and from mechanical impacts.
In other words, the use of HDPE as an anticorrosive coating in critical applications requires the use of a complicated and expensive sandwich technology. For the same reason, the use of HDPE as an inexpensive antierosion coating material is also hampered. Besides, the thermal and chemical resistance of HDPE itself, its compatibility with other polymers, as well as its cost have enough room for further improvements.
It is an object of the present invention to provide a highly filled polymer composition which is inexpensive to manufacture and which combines high deformation capacity with sufficient tensile strength.
It is also an object of the present invention to provide highly filled polymer compositions which have significantly reduced shrinkage and coefficient of thermal expansion, improved chemical and UV resistance, and reduced permeability to oxygen and other corrosive substances.
Furthermore, it is also an object of the present invention to provide highly filled polymer compositions which have highly improved adhesion with other materials, such as metals, and can withstand occasional surface impacts, so that these compositions can be used as single-layer anticorrosive coatings in the manufacture of metal pipes and similar articles.
These and other objects and features of the invention will become more apparent after the consideration of the ensuing description of the invention.